1. Field of the Invention
This invention relates generally to X-ray tubes and is concerned more particularly with a rotatable X-ray target having focal track means for dissipating heat.
2. Discussion of the Prior Art
Generally, a rotating anode X-ray tube comprises a tubular envelope having therein an electron emitting cathode disposed to beam high energy electrons onto a spaced anode target. The target may comprise an axially rotatable disc having adjacent its outer periphery an annular focal track made of an efficient X-ray emitting material, such as tungsten, for example. Thus, electrons beamed from the cathode may be focused onto a focal spot area of the focal track to penetrate into the underlying material and generate X-rays which radiate therefrom and out of the tube.
Most of the electron energy incident on the focal spot area of the focal track is converted to heat energy which could become excessive and damage the surface of the focal track. Consequently, the target disc is rotated at a suitable high angular velocity, such as ten thousand revolutions per minute, for example, to move successive segments of the annular focal track rapidly through the focal spot area aligned with the electron beam. Thus, a one millimeter wide focal spot area on the focal track of a four inch diameter target disc would have successive segments of one millimeter width aligned with the electron beam for only about twenty microseconds, for example.
The penetration depth of an incident electron into the focal track material in the focal spot area is dependent upon the kinetic energy of the electron and the density of the focal track material. Consequently, when the focal track is made of relatively high density material, such as tungsten, for example, the incident electrons penetrate into only a thin layer of the focal track material adjacent the bombarded surface thereof. Thus, electrons having respective energies of about eighty thousand electron volts penetrate into tungsten material to a depth of only about five micrometers, for example.
As a result, the focal track may comprise a thin layer of high density material, such as tungsten-rhenium alloy, for example, disposed annularly on the electron bombarded surface portion of a rotatable disc made of relatively low density material, such as graphite, for example. Thus, the low density material of the substrate disc reduces the inertia of the target and aids in attaining the desired high angular velocity in a relatively shorter time interval, as compared to a disc made of high density material, such as tungsten, for example. Also, the layer of high density material may be provided with an optimum thinness for the low density material of the disc to function as an efficient heat sink in dissipating heat from the focal spot area of the focal track.
However, it has been found difficult to provide a reliable X-ray target having a thin layer of high density material deposited on a disc of low density material. Unless the deposition process is carefully controlled, peel-off and other deteriorating effects may be caused by the thermomechanical stresses developed in rotating anode targets. Also, the sudden transition from the high density material of the layer to the low density material of the disc may cause fracture to occur at the sharp interface.
Therefore, it is advantageous and desirable to provide an X-ray tube with a rotating anode having focal track means for dissipating heat from the focal spot area and avoiding the thermomechanical difficulties encountered in similar tubes of the prior art.